A standard toilet has a bowl and at a location somewhat above the bowl a tank that normally is full of water that can be released into the bowl to flush the bowl, either through direct displacement or siphon action The tank has a floor with an outlet port that is normally blocked by a ring mounted on an overflow tube whose upper end is above the normal liquid level in the tank. For flushing the overflow tube is raised, thereby allowing the water in the tank to rush out the outlet port while a float attached to the overflow tube holds it up off the outlet port until the tank is substantially empty, whereupon the tube reseats itself and the tank is slowly refilled by a float-controlled fill valve.
Such systems are relatively failure prone. The seal at the outlet port can degrade and allow some leakage with the concomitant waste of water, or it can fail altogether 80 that the tank cannot fill. The overflow tube must align perfectly with the outlet port so that any problem with the mechanism can result in the flush valve not closing properly.